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What determines gender balance?

Looking at our human population we know that about half of us are female and half of us are male. The way this
1:1 sex ratio is determined is by genetic means, you get an X from your mum and either an X or Y from your dad;
meaning there is an equal chance to be a female (XX) or male (XY). This seems to be a good way to achieve an even sex
ratio. However quite a few animals go about a far more daring way to determine the sex of their offspring. They let the environment
do it — in particular by using the temperature.

Why are animals letting the sex of their offspring be determined by temperature?

This variable element determines the sex of all crocodiles, and most reptiles and turtles. And if the temperature over a long
period of time doesn’t seem variable enough to you, it is actually the particular temperature during the incubation of the
egg which makes all the decisions. This risky and bizarre mechanism is known as temperature-dependent sex determination
(TSD). It is something the enchanting loggerhead turtle goes through; with cooler temperatures producing more or all males
and warmer temperatures producing more or all females — there is however, also a ‘pivotal temperature’ which produces an
even sex ratio. For the loggerhead turtle it is therefore not only the time of year the female lays her eggs which is crucial,
but also the way she does it — in this way nests on the open beach versus beneath vegetation produce hugely different sex
ratios.

Loggerhead turtles are an endangered species and a balanced sex ratio would help them survive. Photo, Upendra Kanda

So how can such an unusual character work? In truth, we are not so sure — but that is simply what makes it all the more
exciting. There are suggestions that certain enzymes involved in female or male development may only work at particular
temperatures. Or that particular temperatures may result in signals which produce hormones, these in turn result in either
female or male development.

Overlooking the exact mechanics of what is going on, the crucial question I can hear you asking, is why? Why are animals
letting the sex of their offspring be determined by temperature? And whether you may find it intriguing or aggravating, again
the answer is uncertain. Nonetheless, theories and research have given us some direction.

One theory is that TSD may have evolved due to it increasing the group fitness. This involves individuals in the population
being aware that laying eggs at different times of the year or in different ways will influence the sex of their offspring, and so
using it to their advantage. However there hasn’t been any evidence in females changing their behaviour to decide the sex
of their offspring; therefore leading us to a dead end.

Inbreeding provides another theory; due to offspring from parents being all female or all male means that inbreeding is
greatly reduced. The effects of inbreeding are very bad and can lead to a huge fitness loss. This is because it results in a
reduction of genetic variation and so makes it more likely that two undesirable characteristics are brought together. There,
however, has been little research into this theory, making it hard to know how much we can rely on it.
There are also theories that TSD may not be an adaptation at all. This involves TSD coming up in evolution a long time
ago and not conveying an advantage or disadvantage, this effectively neutral trait therefore was not acted on by negative
selection and so still exists today. A problem with this theory is that TSD appears to have evolved multiple times; something
which demands it to be advantageous.

A proposal which makes more sense whilst also being accompanied with evidence, is that TSD may provide individual fitness
benefits; in which particular temperatures are better for one sex. The Agamid lizards provide us with an example of this case;
here eggs laid in the cooler early part of the year tend to be male-biased, this benefits the males as allows them to obtain
more growth prior to hibernation. In this short-lived species this is essential as allows the males to reach reproductive maturity
much sooner, and so be able to reproduce quicker and be more successful.

So why does this all matter? Whether or not you are getting sick of the two seemingly most used words by scientists, politicians
and even your local weather person; global warming is still striking up new-found problems.

Warmer sand temperatures may result in a skewed female-biased sex ratio in turtle populations, causing problematic results
of finding a mate or indeed reducing the variety to choose the best one. Global warming also causing climate change results
in the reverse of the prior problem, which is that increased rainfall may have marked cooling effects on nests, fewer females
therefore means fewer offspring can be produced. You may be thinking, yes turtle populations are known for their endangerment
but also for the high amount of conservation efforts. Despite this being true, research has shown that little concern on
the temperature of incubation has been given in artificial hatcheries. This has resulted in conservation practices producing
highly skewed sex ratios. And so as the conservationists are beaming with smiles as the small ones propel themselves to
the sea all due to their hard efforts, their minds are naively blank with the possibility that they may be doing more harm than
good.

Rather than throwing our hands up in despair, of what seems to be an endless circle of problems after problems, we may
be able to draw a line of hope. What appears to be the answer is a combination of scientific and conservation work, and
importantly communication between the two. By finding out the pivotal temperature of producing an even sex ratio we can
be sure that our next photogenic release can have the snappy caption of ‘saving the world: one female, one male at a time’.